Evidence was obtained that anaerobic organisms isolated from human feces are capable of effectively metabolizing bile acids and other steroidal acids. It was shown that the fecal anaerobes can (a) aromatize the steroidal nucleus to yield estrogens and naphthalenic analogs, (b) cleave the side chain of C24-acids to C22-acids and C19-androgens, and (c) introduce double bonds in the steroidal nuclei. By implication, anaerobic aromatization of steroids by the fecal (anaerobes) should differ from the known aerobic mechanism of biosynthesis of estrogen in mammals. It was inferred by others that the dehydrogenation and aromatization processes of steroids may be a measure of the formation of pro-carcinogens, co-carcinogens or carcinogens in the system. The understanding of the mechanisms of the aromatization process could lead to the development of anti-cancer agents. Our objective is to study the metabolic pathway of anaerobic biosynthesis of estrogens.